For reduction in weight of an automobile, efforts are advanced to increase the strength of a steel material used for an automobile body and to reduce the weight of steel material used. In a thin steel sheet widely used for the automobile, press formability thereof generally decreases with an increase in strength, making it difficult to manufacture a component having a complicated shape. For example, a highly processed portion fractures with a decrease in ductility, and springback becomes prominent to deteriorate dimensional accuracy. Accordingly, it is difficult to manufacture components by performing press-forming on a high-strength steel sheet, in particular, a steel sheet having a tensile strength of 980 MPa or more. It is easy to process the high-strength steel sheet not by press-forming but by roll-forming, but its application target is limited to a component having a uniform cross section in a longitudinal direction.
Methods called hot pressing intended to obtain high formability in the high-strength steel sheet are described in Patent Literatures 1 and 2. By the hot pressing, it is possible to form the high-strength steel sheet with high accuracy to obtain a high-strength hot-pressed steel sheet member.
On the other hand, the hot-pressed steel sheet member is also required to be improved in crashworthiness when the hot-pressed steel sheet member is used for an automobile. The crashworthiness can be improved to some extent by an improvement in ductility. However, steel structure of the steel sheet obtained by the methods described in Patent Literatures 1 and 2 is substantially a martensite single phase, and thus it is difficult for the methods to improve in ductility.
High-strength hot-pressed steel sheet members intended to improve in ductility are described in Patent Literatures 3 to 5, but it is difficult for these conventional hot-pressed steel sheet members to obtain a sufficient crashworthiness. Techniques related to hot pressing are described also in Patent Literatures 6 to 8, but these are also difficult to obtain a sufficient crashworthiness.